A study spanning over 23 different cancer types and around 200,000 twins suggests that a twin sibling who developed cancer increases the risk of the other twin to have any form of cancer. The paper is published in JAMA (Journal of the American Medical Association).

A team of researchers from the Harvard T.H. Chan School of Public Health, the University of Southern Denmark, and the University of Helsinki, provides family risk estimates for both common types of cancer such as breast cancer and rarer ones like head and neck cancer. The findings were generated after studying 23 different forms of cancer among more than 200,000 twins (identical and non-identical) from Denmark, Finland, Norway, and Sweden, of whom 1 in 3 participants had developed cancer over the course of the study.

3,316 pairs entailed cases where both twins had developed the disease; the same cancer was identified in 38 % of identical twins of this group and 26% of the fraternal ones. As per the estimates of the team, co-twins of fraternal pairs had a 37 % risk of having cancer when the other twin was diagnosed with cancer while identical twins had a 46 % risk for the same case.

Furthermore, testicular cancer was found to have the strongest familial risks: the risk of having this cancer type was 12 times greater for a twin if his fraternal twin had developed it, and it was 28 times higher in case of identical twins.

Another finding is that in cases where both twins have developed cancer, each of them has had a different cancer type – this implies a shared increased risk for any type of the disease.

Yet another aspect of the study deals with the heritability of cancer. The large sample has allowed the researchers to evaluate the influence of inherited characteristics in cancer. In general, cancer heritability was found to be 33 %. For some cancer types, the figure was even higher: 58 % for skin melanoma, 57% for prostate cancer, 43 % for non-melanoma skin cancer, 39 % for ovarian cancer, 38 % for kidney cancer (38%), 31 % for breast cancer, and 27 % for uterine cancer. Co-senior author of the study, Jaakko Kaprio, from the University of Helsinki, says that these findings might help in fields like patient counselling and education pertaining to cancer risk.